I suppose you talk about Master Slider Effect. OK. Why it zooming, I think the answer is in nature of rotation around Z, do you remember how Z rotation matrix looks? OK, sorry. Master Effect button simply enable or disable effect located inside the button. Slider does the morph between incoming and outgoing frame, a frame before effect and after effects. This is a uniform way. Try it with some position, resize or recolor effect. It works great. In case of Z rotation there is some zoom-looking. It is one of fun effects.... btw, one more fun thing. Take a look at XYZ matrix key effect, as well as RGB matrix. RGB matrix is a bit tricky at start, but has simple nature. New Red = Red * A + Green * B + Blue * C + Offset. It allow to mix, patch or recolor.